DBMS SHORT QUESTION AND ANSWER
Q1. What is SQL?
Ans: Structured Query Language
2. What is
database?
A database is a logically coherent
collection of data with some inherent meaning, representing some aspect of real
world and which is designed, built and populated with data for a specific
purpose.
3. What is
DBMS?
It is a collection of
programs that enables user to create and maintain a database. In other words it
is general-purpose software that provides the users with the processes of defining, constructing and manipulating
the database for various applications.
4. What is a
Database system?
The database and DBMS
software together is called as Database system.
5. Advantages
of DBMS?
Ø Redundancy is controlled.
Ø Unauthorised access is
restricted.
Ø Providing multiple user
interfaces.
Ø Enforcing integrity
constraints.
Ø Providing backup and recovery.
6.
Disadvantage in File Processing System?
Ø Data redundancy &
inconsistency.
Ø Difficult in accessing
data.
Ø Data isolation.
Ø Data integrity.
Ø Concurrent access is not
possible.
Security
Problems
1.
Describe the three levels
of data abstraction?
The are three levels of
abstraction:
Ø Physical level: The lowest level of abstraction describes how data are
stored.
Ø Logical
level: The
next higher level of abstraction, describes what data are stored in database
and what relationship among those data.
Ø View
level: The
highest level of abstraction describes only part of entire database.
2. Define the
"integrity rules"
There
are two Integrity rules.
Ø Entity Integrity: States
that “Primary key cannot have NULL value”
Ø Referential
Integrity:
States that “Foreign Key can be either a NULL
value or should be Primary Key value of other relation.
3. What is extension
and intension?
Extension
-
It
is the number of tuples present in a table at any instance. This is time
dependent.
Intension
-
It
is a constant value that gives the name, structure of table and the constraints
laid on it.
4. What is System R?
What are its two major subsystems?
System
R was designed and developed over a period of 1974-79 at IBM San Jose
Research Center
Its two subsystems are
Ø Research
Storage
Ø System
Relational Data System.
5. How is the data
structure of System R different from the relational structure?
Unlike
Relational systems in System R
Ø Domains
are not supported
Ø Enforcement
of candidate key uniqueness is optional
Ø Enforcement
of entity integrity is optional
Ø Referential
integrity is not enforced
6. What
is Data Independence
Data
independence means that “the application is independent of the storage
structure and access strategy of data”. In other words, The ability to modify
the schema definition in one level should not affect the schema definition in
the next higher level.
Two types of Data
Independence:
Ø Physical Data Independence
Ø Logical Data Independence
NOTE: Logical Data Independence
7. What is a view? How
it is related to data independence?
A
view may be thought of as a virtual table, that is, a table that does not
really exist in its own right but is instead derived from one or more
underlying base table. In other words, there is no stored file that direct
represents the view instead a definition of view is stored in data dictionary.
Growth
and restructuring of base tables is not reflected in views. Thus the view can
insulate users from the effects of restructuring and growth in the database.
Hence accounts for logical data independence.
13.
What is Data Model?
A collection of conceptual tools for
describing data, data relationships data semantics and constraints.
14.
What is E-R model?
This data model is based on
real world that consists of basic objects called entities and of relationship
among these objects. Entities are described in a database by a set of
attributes.
15.
What is Object Oriented
model?
This model is based on
collection of objects. An object contains values stored in instance variables
with in the object. An object also
contains bodies of code that operate on the object. These bodies of code are
called methods. Objects that contain same types of values and the same methods
are grouped together into classes.
16.
What is an Entity?
It is a 'thing' in the real world with
an independent existence.
17.
What is an Entity type?
It is a collection (set) of
entities that have same attributes.
18.
What is an Entity set?
It is a collection of all
entities of particular entity type in the database.
16.
What is an Extension of
entity type?
The collections of entities
of a particular entity type are grouped together into an entity set.
17.
What is Weak Entity set?
An entity set may not have
sufficient attributes to form a primary key, and its primary key compromises of
its partial key and primary key of its parent entity, then it is said to be
Weak Entity set.
18.
What is an attribute?
It is a particular
property, which describes the entity.
19.
What is a Relation Schema
and a Relation?
A relation Schema denoted
by R(A1, A2, …, An) is made up of the relation name R and the list of
attributes Ai that it contains. A relation is defined as a set of tuples. Let r
be the relation which contains set tuples (t1, t2, t3, ..., tn). Each tuple is
an ordered list of n-values t=(v1,v2, ..., vn).
20.
What is degree of a
Relation?
It is the number of
attribute of its relation schema.
21.
What is Relationship?
It is an association among
two or more entities.
22.
What is Relationship set?
The collection (or set) of
similar relationships.
16.
What is Relationship
type?
Relationship type defines a
set of associations or a relationship set
among a given set of entity types.
17.
What is degree of
Relationship type?
It is the number of entity
type participating.
25.
What is DDL (Data
Definition Language)?
A data base schema is
specifies by a set of definitions expressed by a special language called DDL.
26.
What is VDL (View
Definition Language)?
It specifies user views and
their mappings to the conceptual schema.
27.
What is SDL (Storage
Definition Language)?
This language is to specify
the internal schema. This language may specify the mapping between two schemas.
28.
What is Data Storage -
Definition Language?
The storage structures and
access methods used by database system are specified by a set of definition in
a special type of DDL called data storage-definition language.
29.
What is DML (Data
Manipulation Language)?
This language that enable
user to access or manipulate data as organised by appropriate data model.
Ø Procedural
DML or Low level: DML
requires a user to specify what data are needed and how to get those data.
Ø Non-Procedural
DML or High level: DML requires a user to
specify what data are needed without specifying how to get those data.
31.
What is DML Compiler?
It translates DML
statements in a query language into low-level instruction that the query
evaluation engine can understand.
What is Pl-SQL ?
Procedural Language Structured Query Language
31.
What is Query evaluation engine?
It executes low-level
instruction generated by compiler.
32.
What is DDL Interpreter?
It interprets DDL
statements and record them in tables containing metadata.
34.
What is Record-at-a-time?
The Low level or Procedural
DML can specify and retrieve each record from a set of records. This retrieve
of a record is said to be Record-at-a-time.
35.
What is Set-at-a-time or
Set-oriented?
The High level or
Non-procedural DML can specify and retrieve many records in a single DML
statement. This retrieve of a record is said to be Set-at-a-time or
Set-oriented.
36.
What is Relational Algebra?
It is procedural query
language. It consists of a set of operations that take one or two relations as
input and produce a new relation.
37.
What
is Relational Calculus?
It
is an applied predicate calculus specifically tailored for relational databases
proposed by E.F. Codd. E.g. of languages based on it are DSL ALPHA, QUEL.
34.
How
does Tuple-oriented relational calculus differ from domain-oriented
relational calculus
The
tuple-oriented calculus uses a tuple variables i.e., variable whose only
permitted values are tuples of that relation. E.g. QUEL
The
domain-oriented calculus has domain variables i.e., variables that range over
the underlying domains instead of over relation. E.g. ILL, DEDUCE.
35.
What
is normalization?
It is a process of
analysing the given relation schemas based on their Functional Dependencies
(FDs) and primary key to achieve the properties
Ø Minimizing redundancy
Ø Minimizing insertion,
deletion and update anomalies.
36.
What is Functional
Dependency?
A Functional dependency is denoted by X Y between two sets of attributes X and Y
that are subsets of R specifies a constraint on the possible tuple that can
form a relation state r of R. The constraint is for any two tuples t1 and t2 in
r if t1[X] = t2[X] then they have t1[Y] = t2[Y]. This means the value of X
component of a tuple uniquely determines the value of component Y.
37.
When is a functional
dependency F said to be minimal?
Ø Every dependency in F has a
single attribute for its right hand side.
Ø 
We cannot replace any dependency X A in F with a dependency Y A where Y is a proper subset of X and still
have a set of dependency that is equivalent to F.
We cannot replace any dependency X A in F with a dependency Y A where Y is a proper subset of X and still
have a set of dependency that is equivalent to F.
Ø We cannot remove any
dependency from F and still have set of dependency that is equivalent to F.
38.
What is Multivalued
dependency?
Multivalued dependency denoted by X
Y specified on relation schema R, where X and Y are both subsets of R,
specifies the following constraint on any relation r of R: if two tuples t1 and
t2 exist in r such that t1[X] = t2[X] then t3 and t4 should also exist in r
with the following properties
Ø t3[x] = t4[X] = t1[X] =
t2[X]
Ø t3[Y] =
t1[Y] and t4[Y] = t2[Y]
Ø t3[Z] =
t2[Z] and t4[Z] = t1[Z]
where [Z = (R-(X U Y)) ]
39.
What is Lossless join
property?
It guarantees that the
spurious tuple generation does not occur with respect to relation schemas after
decomposition.
34. What is 1 NF (Normal Form)?
The domain of attribute must include only atomic (simple,
indivisible) values.
35. What is Fully Functional
dependency?
It is based on
concept of full functional dependency. A functional dependency X
Y is full functional dependency if removal of any attribute A from X
means that the dependency does not hold any more.
36. What is 2NF?
A relation schema R is in 2NF if it is in 1NF and every non-prime
attribute A in R is fully functionally dependent on primary key.
37. What is 3NF?
A relation schema R
is in 3NF if it is in 2NF and for every FD X
A either of the following is true
Ø X is a Super-key of R.
Ø A is a prime attribute of R.
In other words, if every non prime attribute is non-transitively
dependent on primary key.
38. What is BCNF (Boyce-Codd
Normal Form)?
A relation schema R is in BCNF if it
is in 3NF and satisfies an additional constraint that for every FD X A, X must be a candidate key.
39. What is 4NF?
A relation schema R
is said to be in 4NF if for every Multivalued dependency X Y that holds over R, one of following
is true
Ø X is subset or equal to (or) XY = R.
X is a super key
34. What is 5NF?
A Relation schema R is said to be 5NF if for every join
dependency {R1, R2, ..., Rn} that holds
R, one the following is true
Ø Ri = R for some i.
Ø The join dependency is implied by the set of FD, over R in which the
left side is key of R.
51. What is Domain-Key Normal Form?
A relation is said to be in DKNF if all constraints and dependencies
that should hold on the the constraint can be enforced by simply enforcing the
domain constraint and key constraint on the relation.
52. What are
partial, alternate,, artificial, compound and natural key?
Partial Key:
It is a set of attributes that can uniquely identify weak entities
and that are related to same owner entity. It is sometime called as Discriminator.
Alternate Key:
All
Candidate Keys excluding the Primary Key are known as Alternate Keys.
Artificial Key:
If no obvious key, either stand alone or
compound is available, then the last resort is to simply create a key, by
assigning a unique number to each record or occurrence. Then this is known as
developing an artificial key.
Compound Key:
If no single data element
uniquely identifies occurrences within a construct, then combining multiple
elements to create a unique identifier for the construct is known as creating a
compound key.
Natural Key:
When one of the data elements stored
within a construct is utilized as the primary key, then it is called the
natural key.
53. What is indexing
and what are the different kinds of indexing?
Indexing
is a technique for determining how quickly specific data can be found.
Types:
Ø Binary
search style indexing
Ø B-Tree
indexing
Ø Inverted
list indexing
Ø Memory
resident table
Ø Table
indexing
52. What is system
catalog or catalog relation? How is better known as?
A
RDBMS maintains a description of all the data that it contains, information
about every relation and index that it contains. This information is stored in
a collection of relations maintained by the system called metadata. It is also
called data dictionary.
53. What is meant by
query optimization?
The
phase that identifies an efficient execution plan for evaluating a query that
has the least estimated cost is referred to as query optimization.
54. What is join
dependency and inclusion dependency?
Join Dependency:
A Join dependency is generalization of Multivalued
dependency.A JD {R1, R2, ..., Rn} is said to hold over a relation R if R1, R2,
R3, ..., Rn is a lossless-join decomposition of R . There is no set of sound
and complete inference rules for JD.
Inclusion Dependency:
An Inclusion Dependency
is a statement of the form that some columns of a relation are contained in
other columns. A foreign key constraint is an example of inclusion dependency.
55. What is
durability in DBMS?
Once
the DBMS informs the user that a transaction has successfully completed, its
effects should persist even if the system crashes before all its changes are
reflected on disk. This property is called durability.
56. What do you mean
by atomicity and aggregation?
Atomicity:
Either
all actions are carried out or none are. Users should not have to worry about
the effect of incomplete transactions. DBMS ensures this by undoing the actions
of incomplete transactions.
Aggregation:
A concept which is used to model a
relationship between a collection of entities and relationships. It is used
when we need to express a relationship among relationships.
57. What is a
Phantom Deadlock?
In
distributed deadlock detection, the delay in propagating local information
might cause the deadlock detection algorithms to identify deadlocks that do not
really exist. Such situations are called phantom deadlocks and they lead to
unnecessary aborts.
58. What is a
checkpoint and When does it occur?
A
Checkpoint is like a snapshot of the DBMS state. By taking checkpoints, the
DBMS can reduce the amount of work to be done during restart in the event of subsequent
crashes.
52. What are the
different phases of transaction?
Different
phases are
Ø Analysis
phase
Ø Redo
Phase
Ø Undo
phase
53. What do you mean
by flat file database?
It
is a database in which there are no programs or user access languages. It has
no cross-file capabilities but is user-friendly and provides user-interface
management.
54. What is
"transparent DBMS"?
It
is one, which keeps its Physical Structure hidden from user.
55. Brief theory of
Network, Hierarchical schemas and their properties
Network
schema uses a graph data structure to organize records example for such a
database management system is CTCG while a hierarchical schema uses a tree data
structure example for such a system is IMS.
56. What is a query?
A
query with respect to DBMS relates to user commands that are used to interact
with a data base. The query language can be classified into data definition
language and data manipulation language.
57. What do you mean
by Correlated subquery?
Subqueries, or nested queries, are used to bring back a set of rows
to be used by the parent query. Depending on how the subquery is written, it
can be executed once for the parent query or it can be executed once for each
row returned by the parent query. If the subquery is executed for each row of
the parent, this is called a correlated
subquery.
A correlated subquery can be easily identified if it contains any
references to the parent subquery columns in its WHERE clause. Columns from the
subquery cannot be referenced anywhere else in the parent query. The following
example demonstrates a non-correlated subquery.
E.g. Select * From CUST Where '10/03/1990' IN
(Select ODATE From ORDER Where CUST.CNUM = ORDER.CNUM)
58. What are the
primitive operations common to all record management systems?
Addition, deletion and modification
52. Name the buffer
in which all the commands that are typed in are stored
‘Edit’
Buffer
53. What are the
unary operations in Relational Algebra?
PROJECTION
and SELECTION.
54. Are the
resulting relations of PRODUCT and JOIN operation the same?
No.
PRODUCT:
Concatenation of every row in one relation with every row in another.
JOIN:
Concatenation of rows from one relation and related rows from another.
55. What is RDBMS
KERNEL?
Two
important pieces of RDBMS architecture are the kernel, which is the software,
and the data dictionary, which consists of the system-level data structures
used by the kernel to manage the database
You might think of an RDBMS as an
operating system (or set of subsystems), designed specifically for controlling
data access; its primary functions are storing, retrieving, and securing data.
An RDBMS maintains its own list of authorized users and their associated
privileges; manages memory caches and paging; controls locking for concurrent
resource usage; dispatches and schedules user requests; and manages space usage
within its table-space structures
.
56. Name the
sub-systems of a RDBMS
I/O,
Security, Language Processing, Process Control, Storage Management, Logging and
Recovery, Distribution Control, Transaction Control, Memory Management, Lock
Management
57. Which part of
the RDBMS takes care of the data dictionary? How
Data
dictionary is a set of tables and database objects that is stored in a special
area of the database and maintained exclusively by the kernel.
58. What is the job
of the information stored in data-dictionary?
The information in the data dictionary validates the
existence of the objects, provides access to them, and maps the actual physical
storage location.
52. Not
only RDBMS takes care of locating data it also
Not
only RDBMS takes care of locating data it also
determines
an optimal access path to store or retrieve the data
53. How do you
communicate with an RDBMS?
You
communicate with an RDBMS using Structured Query Language (SQL)
54. Define SQL and
state the differences between SQL and other conventional programming Languages
SQL
is a nonprocedural language that is designed specifically for data access
operations on normalized relational database structures. The primary difference
between SQL and other conventional programming languages is that SQL statements
specify what data operations should be performed rather than how to perform
them.
55. Name the three
major set of files on disk that compose a database in Oracle
There are three major sets of files on disk that compose a database.
All the files are binary. These are
Ø Database files
Ø Control files
Ø Redo logs
The most important of these are the database files where the actual
data resides. The control files and the redo logs support the functioning of
the architecture itself.
All three sets of files must be present, open, and available to
Oracle for any data on the database to be useable. Without these files, you
cannot access the database, and the database administrator might have to
recover some or all of the database using a backup, if there is one.
56. What is an Oracle
Instance?
The Oracle system processes, also known as Oracle background
processes, provide functions for the user processes—functions that would
otherwise be done by the user processes themselves
Oracle database-wide system memory is known as the SGA, the system global area or shared global area. The data and control
structures in the SGA are shareable, and all the Oracle background processes
and user processes can use them.
The combination of the SGA and the Oracle background processes is
known as an Oracle instance
57. What are the
four Oracle system processes that must always be up and running for the
database to be useable
The four Oracle system processes that must always be up and running
for the database to be useable include DBWR
(Database Writer), LGWR (Log Writer), SMON (System Monitor), and PMON
(Process Monitor).
Database Files
The database files hold the actual data and are typically the
largest in size. Depending on their sizes, the tables (and other objects) for
all the user accounts can go in one database file—but that's not an ideal
situation because it does not make the database structure very flexible for
controlling access to storage for different users, putting the database on
different disk drives, or backing up and restoring just part of the database.
You
must have at least one database file but usually, more than one files are used.
In terms of accessing and using the data in the tables and other objects, the
number (or location) of the files is immaterial.
The database files are fixed in size and never grow bigger than the
size at which they were created
Control Files
The control files and redo logs support the rest of the
architecture. Any database must have at least one control file, although you
typically have more than one to guard against loss. The control file records
the name of the database, the date and time it was created, the location of the
database and redo logs, and the synchronization information to ensure that all
three sets of files are always in step. Every time you add a new database or
redo log file to the database, the information is recorded in the control
files.
Redo Logs
Any database must have at least two redo logs. These are the
journals for the database; the redo logs record all changes to the user objects
or system objects. If any type of failure occurs, the changes recorded in the
redo logs can be used to bring the database to a consistent state without
losing any committed transactions. In the case of non-data loss failure, Oracle
can apply the information in the redo logs automatically without intervention
from the DBA.
The redo log files are fixed in size and never grow dynamically from
the size at which they were created.
53. What is ROWID?
The ROWID is a unique database-wide physical address for every row
on every table. Once assigned (when the row is first inserted into the
database), it never changes until the row is deleted or the table is dropped.
The ROWID consists of the following three components, the
combination of which uniquely identifies the physical storage location of the
row.
Ø Oracle database file number, which contains the block with the rows
Ø Oracle block address, which contains the row
Ø The row within the block (because each block can hold many rows)
The ROWID is used internally in indexes as a quick means of
retrieving rows with a particular key value. Application developers also use it
in SQL statements as a quick way to access a row once they know the ROWID
54. What is Oracle
Block? Can two Oracle Blocks have the same address?
Oracle "formats" the database files into a number of
Oracle blocks when they are first created—making it easier for the RDBMS
software to manage the files and easier to read data into the memory areas.
The block size should be a multiple of the operating system block
size. Regardless of the block size, the entire block is not available for
holding data; Oracle takes up some space to manage the contents of the block.
This block header has a minimum size, but it can grow.
These Oracle blocks are the smallest unit of storage. Increasing the
Oracle block size can improve performance, but it should be done only when the
database is first created.
Each Oracle block is numbered sequentially for each database file
starting at 1. Two blocks can have the same block address if they are in
different database files.
55. What is database
Trigger?
A database trigger is a PL/SQL block that can defined to
automatically execute for insert, update, and delete statements against a
table. The trigger can e defined to execute once for the entire statement or
once for every row that is inserted, updated, or deleted. For any one table,
there are twelve events for which you can define database triggers. A database
trigger can call database procedures that are also written in PL/SQL.
56. Name two
utilities that Oracle provides, which are use for backup and recovery.
Along with the RDBMS software, Oracle provides two utilities that
you can use to back up and restore the database. These utilities are Export and Import.
The Export utility dumps the definitions and data
for the specified part of the database to an operating system binary file. The Import utility reads the file produced by an export, recreates the
definitions of objects, and inserts the data
If Export and Import are used as a means of backing up and
recovering the database, all the changes made to the database cannot be
recovered since the export was performed. The best you can do is recover the
database to the time when the export was last performed.
52. Name two
utilities that Oracle provides, which are use for backup and recovery.
Along with the RDBMS software, Oracle provides two utilities that
you can use to back up and restore the database. These utilities are Export and Import.
The Export utility dumps the definitions and data
for the specified part of the database to an operating system binary file. The Import utility reads the file produced by an export, recreates the
definitions of objects, and inserts the data
If Export and Import are used as a means of backing up and
recovering the database, all the changes made to the database cannot be
recovered since the export was performed. The best you can do is recover the
database to the time when the export was last performed.
53. What are
stored-procedures? And what are the advantages of using them.
Stored
procedures are database objects that perform a user defined operation. A stored
procedure can have a set of compound SQL statements. A stored procedure
executes the SQL commands and returns the result to the client. Stored
procedures are used to reduce network traffic.
54. Tables derived
from the ERD
a) Are totally unnormalised
b) Are always in 1NF
c) Can be further denormalised
d) May have multi-valued attributes
(b) Are always in 1NF
55. Spurious tuples
may occur due to
i. Bad normalization
ii. Theta joins
iii. Updating tables from join
a) i & ii b) ii & iii
c) i & iii d) ii &
iii
(a) i
& iii because theta joins are joins made on keys that are
not primary keys.
56. A B C is a set
of attributes. The functional dependency is as follows
AB -> B
AC -> C
C -> B
a) is in 1NF
b) is in 2NF
c) is in 3NF
d) is in BCNF
(a) is in 1NF since (AC)+ = { A, B,
C} hence AC is the primary key. Since
C B is a FD given, where
neither C is a Key nor B is a prime attribute, this it is not in 3NF. Further B
is not functionally dependent on key AC thus it is not in 2NF. Thus the given FDs is in 1NF.
52. In mapping of
ERD to DFD
a) entities in ERD should correspond
to an existing entity/store in DFD
b) entity in DFD is converted to
attributes of an entity in ERD
c) relations in ERD has 1 to 1
correspondence to processes in DFD
d) relationships in ERD has 1 to 1
correspondence to flows in DFD
(a)
entities in ERD should correspond to an existing entity/store in DFD
53. A dominant
entity is the entity
a) on the N side in a 1 : N
relationship
b) on the 1 side in a 1 : N
relationship
c) on either side in a 1 : 1
relationship
d) nothing to do with 1 : 1 or 1 : N
relationship
(b)
on the 1 side in a 1 : N relationship
54. Select 'NORTH', CUSTOMER From CUST_DTLS Where REGION
= 'N' Order By
CUSTOMER Union
Select 'EAST', CUSTOMER From CUST_DTLS
Where REGION = 'E' Order By CUSTOMER
The
above is
a) Not an error
b) Error - the string in single
quotes 'NORTH' and 'SOUTH'
c) Error - the string should be in
double quotes
d) Error - ORDER BY clause
(d) Error - the ORDER BY clause. Since ORDER BY
clause cannot be used in UNIONS
55. What is Storage
Manager?
It is a program module that provides the interface between the
low-level data stored in database, application programs and queries submitted
to the system.
56. What is Buffer Manager?
It is a program module, which is responsible for fetching data from
disk storage into main memory and deciding what data to be cache in
memory.
57. What is Transaction
Manager?
It is a program module, which ensures that database, remains in a
consistent state despite system failures and concurrent transaction execution
proceeds without conflicting.
58. What is File Manager?
It is a program module, which manages the allocation of space on
disk storage and data structure used to represent information stored on a
disk.
59. What is Authorization and
Integrity manager?
It is the program module, which tests for the satisfaction of integrity
constraint and checks the authority of user to access data.
60. What are
stand-alone procedures?
Procedures that are not part of a package are known as stand-alone
because they independently defined. A good example of a stand-alone procedure
is one written in a SQL*Forms application. These types of procedures are not
available for reference from other Oracle tools. Another limitation of
stand-alone procedures is that they are compiled at run time, which slows
execution.
61. What are cursors
give different types of cursors.
PL/SQL uses
cursors for all database information accesses statements. The language supports
the use two types of cursors
Ø Implicit
Ø Explicit
62. What is cold
backup and hot backup (in case of Oracle)?
Ø Cold Backup:
It is copying the three
sets of files (database files, redo logs, and control file) when the instance
is shut down. This is a straight file copy, usually from the disk directly to
tape. You must shut down the instance to guarantee a consistent copy.
If a cold backup is performed, the only option available in the
event of data file loss is restoring all the files from the latest backup. All
work performed on the database since the last backup is lost.
Ø Hot Backup:
Some sites (such as worldwide airline reservations systems) cannot
shut down the database while making a backup copy of the files. The cold backup
is not an available option.
So different means of backing up database must be used — the hot
backup. Issue a SQL command to indicate to Oracle, on a tablespace-by-tablespace
basis, that the files of the tablespace are to backed up. The users can
continue to make full use of the files, including making changes to the data.
Once the user has indicated that he/she wants to back up the tablespace files,
he/she can use the operating system to copy those files to the desired backup
destination.
The database must be running in ARCHIVELOG mode for the hot backup
option.
If a data loss failure does occur, the lost database files can be
restored using the hot backup and the online and offline redo logs created
since the backup was done. The database is restored to the most consistent
state without any loss of committed transactions.
63. What are
Armstrong rules? How do we say that they are complete and/or sound
The well-known inference rules for FDs
Ø Reflexive rule :
If Y is subset or equal to X then X Y.
Ø Augmentation rule:
If X Y then XZ YZ.
Ø Transitive rule:
If {X Y, Y
Z} then X Z.
Ø Decomposition rule :
If X YZ then X Y.
Ø Union or
Additive rule:
If {X Y, X Z} then X YZ.
Ø Pseudo Transitive rule :
If {X Y, WY Z}
then WX Z.
Of these the first three are known
as Amstrong Rules. They are sound because it is enough if a set of FDs satisfy
these three. They are called complete because using these three rules we can
generate the rest all inference rules.
64. How can you find
the minimal key of relational schema?
Minimal
key is one which can identify each tuple of the given relation schema uniquely.
For finding the minimal key it is required to find the closure that is the set
of all attributes that are dependent on any given set of attributes under the
given set of functional dependency.
Algo.
I Determining X+, closure for X,
given set of FDs F
1.
Set X+
= X
2.
Set Old X+
= X+
3.
For each
FD Y Z in F and if Y belongs to X+ then add Z to X+
For each
FD Y Z in F and if Y belongs to X+ then add Z to X+
4.
Repeat steps 2
and 3 until Old X+ = X+
Algo.II Determining
minimal K for relation schema R, given set of FDs F
1.
Set K to R that
is make K a set of all attributes in R
2.
For each
attribute A in K
a.
Compute (K – A)+
with respect to F
b.
If (K – A)+ = R then set K = (K – A)+
65. What do you
understand by dependency preservation?
Given
a relation R and a set of FDs F, dependency preservation states that the
closure of the union of the projection
of F on each decomposed relation Ri is equal to the closure of F. i.e.,
((PR1(F)) U … U (PRn(F)))+
= F+
if
decomposition is not dependency preserving, then some dependency is lost in the
decomposition.
66. What is meant by
Proactive, Retroactive and Simultaneous Update.
Proactive
Update:
The
updates that are applied to database before it becomes effective in real world
.
Retroactive
Update:
The
updates that are applied to database after it becomes effective in real world .
Simulatneous
Update:
The
updates that are applied to database at the same time when it becomes effective
in real world .
67. What are the different
types of JOIN operations?
Equi Join: This is the most
common type of join which involves only equality comparisions. The disadvantage
in this type of join is that there
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